1. Field of the Invention
The present invention generally relates to an image signal processor and, more particularly, to a type of image signal processor used in connection with a video output device such as, for example, a television receiver set, a video tape player (VTR) or a video camera. The image signal processor stores signal outputted from the video output device and subsequently writes the stored video signal in a graphic memory of a personal computer in the form of a still or a frozen picture.
2. Description of the Prior Art
With the advent of widespread use of semiconductor memories, numerous image signal processors of the type referred to above have been proposed and has been made commercially available. When the image signal descriptive of one frozen picture is read in the personal computer through the image signal processor, an operator of the personal computer can perform an image analysis of the frozen picture or image processing such as enlargement or reduction of the frozen picture, and extraction or highlighting of one or more portions of the frozen pictures.
According to the prior art, an image signal processor is provided with a memory device for storing digitized image signals. When a WRITE-IN control signal or a READ-OUT control signal is supplied to the memory device, the image signal can be inputted to or outputted from the memory device, respectively.
Generally in the prior art image signal processor, when the image signal stored in the memory device is desired to be read out from the memory device for transfer to the personal computer, the following process takes place. Specifically, after an image signal representative of one frozen picture has been written in the memory device of the image signal processor, the image signal processor transmits a signal to the personal computer notifying the personal computer that the image signal has been stored in the memory device. The personal computer subsequently interrogates the image signal processor if the image signal stored in the memory device can be read out from the memory device. When the image signal processor is so interrogated, a READY signal is transmitted to the personal computer notifying the personal computer that the image signal in the memory device is ready to be read out therefrom. After this interrogation, the image signal is transferred onto the personal computer. The image signal read out from the memory device is then stored in an internal graphic memory in the personal computer.
According to the prior art, in order for the image signal stored in the memory device to be transferred onto the personal computer, a plurality of interrogations must be made between the image signal processor and the personal computer, and a relatively long time is required to complete the transfer of the image signal to the personal computer.
The prior art image signal processor also has another problem associated with the write-in operation of the image signal. Specifically, when the frozen color picture which is composed of, for example, red, green and blue is desired to be reproduced in a color as faithful as possible to the color of the original video image, it is generally recognized that a resolving power of at least 4 to 8 bits (16 to 256 colors) is required for each color of the frozen color picture. This means that the memory device should have a large memory capacity to store a number of color image data. While the price in the market of semiconductor memories has been lowering because of mass-production, the memory device used in the image signal processor is, in practice, composed of a number of memory chips and, therefore, an increased number of the memory chips may result not only in a cost increase of the image signal processor, but also in a size increase of the image signal processor and the associated circuit components.
On the other hand, in the field of facsimile technology in which the input image signal is digitized to provide a frozen picture, a DITHER process is generally used. According to the DITHER process, the input image signal representative of the original image is inputted to a comparator whose threshold value is variable stepwise so that a plurality of digitized images of different gradations can be obtained. The digitized images of different gradations are then properly combined together to provide a single frozen picture having continuously varying gradations.
However, this technique has posed a problem in that the use of a circuit for varying the threshold value is required which tends to make the image signal processor as a whole bulky in size. Also, a process of combining the digitized images together to provide the single frozen picture is complicated, and a real-time accomplishment of the process is hampered.